The invention relates to an A/D conversion device having an analog input intended to receive an analog input voltage, an input intended to receive a regulation signal, and a digital output intended to supply a digital signal resulting from the conversion of the analog input voltage, comprising:
a first amplifier having an output and a first signal input connected to the analog input, a second gain control input intended to receive a voltage allowing modification of the amplifier gain and connected to a power supply terminal via a storage capacitor, PA1 an A/D converter having an analog input connected to the output of the amplifier and an output constituting the output of the device, PA1 a regulation module having an enable input intended to receive an enable signal and comprising a first controllable current source connected to the storage capacitor with which the voltage at the terminals of said capacitor can be varied, and a first comparator intended to perform a comparison between a signal which is representative of the voltage at the output of the first amplifier and the regulation signal, and supplying a signal for controlling the conduction of the current source when the enable signal is in an active state. PA1 a second amplifier similar to the first amplifier, second amplifier whose signal input is intended to be placed at a zero potential and whose gain control input is intended to be placed at a predetermined potential, PA1 a second comparator having an output, a first input connected to the output of the second amplifier and a second input connected to one of the junction points of the resistance ladder, and PA1 a third controllable current source intended to supply a bias current via the resistance ladder, said current having a value which is determined by the value of the voltage at the output of the second comparator.
Such a device is known from European patent application no. 0 271 936 B1 which describes a system for controlling the gain of the first amplifier, based on identification and control of a voltage level of reference thresholds of the output signal of said amplifier, which thresholds correspond to similar thresholds of the input signal. The amplifier gain is subsequently adjusted in such a way that the result of the A/D conversion of its output signal does not exceed a certain value, referred to as maximum digital value. This system has the drawback that it can only function with signals having reference thresholds of the video signal type, which are often altered by electromagnetic noise. The reference thresholds are then not very reliable, which jeopardizes the gain control of the amplifier. Moreover, the A/D conversion of signals having no reference thresholds is not possible with such a device.